A common surgical invention is spine fusion, in which two or more adjacent vertebral bodies are fused together in order to alleviate pain associated with the disc(s) located between those vertebral bodies. While spine fusion generally helps to eliminate certain types of pain, it has also been shown to decrease function by limiting the range of motion for patients in flexion, extension, rotation and lateral bending. Furthermore, it is believed that spine fusion creates increased stresses on adjacent non-fused motion segments. Also, the fusion device used to effect fusion, whether artificial or biological, may migrate out of the fusion site.
Posterior elements called facet joints, the small joints located behind adjacent vertebrae in the spine that allow for spinal motion, help to support axial, torsional and shear loads that act on the spinal column. The facet joints are diarthroidal joints that provide both sliding articulation and load transmission features. The facet's articular surfaces contact in extension, limiting rotation and increasing compressive load. The articular surfaces also contact on one side of the spine in lateral bending and axial rotation, also limiting rotation and transferring load. However, one of the root causes of back pain, particularly the persistent and disabling kind, is facet joints. The articular cartilaginous surfaces can degenerate due to mechanical or biological factors and cause pain as with other joint osteoarthritis. For example, a patient may suffer foam arthritic facet joints, severe facet joint tropism or otherwise deformed facet joints, facet joint injuries, etc. Furthermore, problems with the facet joints can also complicate treatments associated with other portions of the spine. There is currently a lack of suitable intervention procedures for facet joint disorders. Facetectomy, the removal of the facet joints, may provide some relief, but is also believed to significantly decrease the stiffness of the spinal column in all planes of motion.
There are several types of metal facet screws for fusion of facet joints but the metal screws compromise a large surface area of the facet, predisposing the facet to fracture. Consequently, the use of metal screws for fusion of facet joints is risky. The angle of insertion of the metal screw must be perfect. The use of pre-shaped, harvested or synthetic bone as a structural fixation for facet joint fusion offers three distinct advantages over pedicle or compression screws, which are presently used in facet fusion procedures: (1) using bone instead of metal allows for natural bone in-growth and a stronger, permanent fusion; (2) the natural or synthetic graft cannot work its way loose over time, a concern with screw type fixation; and, (3) the graft is self-leveling, which eliminates any concern of vertebral tilting. Tapered allograft dowels are known but there is nothing on those devices to prevent their migration from the place of insertion. Other types of allograft dowels have threads, but the threads tend to break. Interference allograft screws are known but are not suitable for use with facets. A molly bolt-type fusion mechanism with a gun inserter is known but does not provide the surgeon sufficient control when working with the spine.
What is needed, and is not found in the prior art, is an allograft facet fusion system that overcomes the disadvantages of prior art facet fusion systems.